How Do Test Size and Dissolution Modify Stable Isotope Ratios and Mg/Ca in Planktonic Foraminifer Tests? a Quantitative Analysis

Abstract:

We present new data for stable carbon and oxygen isotope ratios in four test-size ranges for Neogloboquadrina dutertrei from the EEP, and from four foraminifer species on RGR: Globorotalia menardii, Globorotalia truncatulinoides, Globorotalia inflata and Globigerinoides conglobatus. The EEP has a steep gradient to environmental parameters allowing statistical analyses of the effect of habitat and test size on shell chemistry. The RGR permits isolating the effect of sedimentary dissolution on shell chemistry because there is a strong gradient to sedimentary calcite dissolution there while surface environmental parameters remain invariable among core tops. Our statistical analyses include ANOVA, matched pair and independent means t-tests, univariate and multiple linear regression, principle components, and geographic distribution analyses. Calcite dissolution is estimated using the G. menardiiFragmentation Index.

Stable isotope data from small N. dutertrei shells (< 350µm) provide more accurate, statistically significant, and consistent estimates of environmental parameters than larger foraminifers. Temperature estimates made from stable oxygen isotopes disagree with those made with Mg/Ca with large and statistically significant differences (up to 4°C) using the same species, in the same core top samples and within the same test-size ranges. Previous studies have shown the strong effect of dissolution in modifying Mg/Ca content of foraminifer tests. While there is no statistically significant evidence on RGR that dissolution modifies stable isotope ratios in foraminifer shells; in EEP core tops we observe a strong and statistically significant relationship between dissolution and shell stable isotope chemistry. Geographic analyses and radiocarbon data suggest that sediments experiencing much dissolution can be as old as 9000 years, and there is a linear and statistically significant relationship between degree of dissolution and core top sediment age. The greater the dissolution, the older the sediment. Therefore, environmental inferences from shell chemistry are not comparable to modern environmental parameters, and dissolution indirectly confounds the accurate interpretation of shell stable isotope data while directly altering the Mg/Ca content of foraminifer shells.